The simplest way is to use a tire size calculator. Lots of websites have them. Here's one
Tire Size Calculator
If you want to know what the numbers mean, the first number is the width in mm of the tire, the second represents the sidewall dimension and is a percentage of the width, the third is the wheel size.
width = 225mm = 8.8"
sidewall = 75% of 225 = 168mm = 6.6"
wheel diameter = 16 + 6.6" sidewall (x2) = 29.2 overall diameter
so you have 29's.. not quite 9" wide